1. Field of the Invention
The invention relates to a vehicle suspension system. More particularly, the invention relates to a vehicle suspension system including four shock absorbers which are provided so as to correspond to a left front wheel, a right front wheel, a left rear wheel and the right rear wheel; and one control cylinder which is connected to these shock absorbers.
2. Description of the Related Art
The above-mentioned type of vehicle suspension system has been known, as disclosed in, for example, U.S. Pat. No. 3,024,037. In a vehicle suspension system disclosed in U.S. Pat. No. 3,024,037, a control cylinder includes a cylinder housing, and a piston assembly which is fitted in the cylinder housing fluid-tightly and slidably. A suspension cylinder is formed by integrating a hydraulic cylinder and an air spring. The piston assembly is formed by coupling two pistons with each other using a coupling rod. A space in the cylinder housing is partitioned into two cylinder chambers by a partition wall. The pistons included in the piston assembly are fitted in respective cylinder chambers fluid-tightly and slidably. The coupling rod is fitted in the partition wall fluid-tightly and slidably so as to penetrate the partition wall. Four fluid chambers thus formed in the cylinder housing are connected to fluid chambers of fluid pressure cylinder portions of four respective suspension cylinders provided so as to correspond to the right and left wheels on the front side. In the control cylinder, an outer side pressure-receiving surface of one of the two pistons is used as a pressure-receiving surface which receives fluid pressure of the suspension cylinder corresponding to the left rear wheel, and an outer side pressure-receiving surface of the other piston is used as a pressure-receiving surface which receives fluid-pressure of the suspension cylinder corresponding to the right rear wheel. Also, an inner side pressure-receiving surface of one of the two pistons is used as a pressure-receiving surface which receives fluid pressure of the suspension cylinder corresponding to the left front wheel, and an inner side pressure-receiving surface of the other piston is used as a pressure-receiving surface which receives fluid pressure of the suspension cylinder corresponding to the right front wheel. Accordingly, for example, when a braking force is applied to a vehicle and pitching moment is applied to a vehicle body, and therefore the fluid pressure of each of the suspension cylinders corresponding to the right and left wheels on the front side is increased and the fluid pressure of each of the suspension cylinders corresponding to the right and left wheels on the rear side is decreased, the piston assembly does not move. Therefore, the four suspension cylinders are substantially independent of each other. As a result, pitching of the vehicle body is suppressed.
When rolling moment is applied to the vehicle body while the vehicle is turning and therefore the fluid pressure of each of the suspension cylinders corresponding to the front and rear wheels on the left side is increased and the fluid pressure of each of the suspension cylinders corresponding to the front and rear wheels on the right side is decreased, the fluid pressure applied to one of the two outer side pressure-receiving surfaces and the fluid pressure applied to one of the two inner side pressure-receiving surfaces are increased, and the fluid pressure applied to the other outer side pressure-receiving surface and the fluid pressure applied to the other inner side pressure-receiving surface are decreased. In this case, whether the piston assembly moves is not decided uniquely. If the piston assembly moves, the direction in which the piston assembly moves is not decided uniquely. Whether the piston assembly moves and the direction in which the piston assembly moves vary depending on a change in the fluid pressure caused in the suspension cylinder due to rolling of the vehicle body and a relationship of size between the outer side pressure-receiving surface and the inner side pressure-receiving surface. Namely, it is possible to make rolling rigidity on the front wheel side different from rolling rigidity on the rear wheel side, for example, by changing the relationship of size between the outer side pressure-receiving surface and the inner side pressure-receiving surface with respect to the same change in the fluid pressure caused in the suspension cylinder due to rolling.
Further, when one of the four wheels runs on a bump on a road, the piston assembly of the control cylinder moves, which makes it easier for the suspension cylinder corresponding to the one wheel to operate. As a result, transfer of the impact that is caused when the one wheel runs on the bump is reduced, which improves the riding comfort.
As described so far, the vehicle suspension system disclosed in U.S. Pat. No. 3,024,037 is good. However, the vehicle suspension system still has plenty of room for improvement.